CRUSHING AND GRINDING 215 parallel to the axis) revolving over a bed plate which is also corrugated. The material is thrown up by the beater wheel over a barrier and then comes around a race course and under the beater again. SIZES AND CAPACITIES OF LARGE Disc CRUSHERS Inches Inches Inches Size of crusher . . 48 11% by 17 8 40 65 36 9J4 by 14K 5 25 40 24 7 by 10% 15 25 Opening in elliptical feed sp Width between discs at feed Capacity, tons per hour to 1, mately. ........... out .......... spout ........ HJ in., approxi- Horsepower required Aids to comminution are of various sorts. Gummy or resinous materials can often be ground when chilled when they resist all attempts at subdivision at ordinary temperatures. Occasionally a material which is to be added at a later time greatly increases the readiness with which some other material can be ground. Practically all the waterproofing agents used with cements can be readily ground after adding some of the cement, where they cannot without it. The excipients of the druggists come in this category. FIG. 11.—Longitudinal section of beater. Work Done in Rock Crushing.—It is important, in order to compare the work accomplished by various machines, to have some basis upon which such comparisons may be founded; There are two basic laws that have had attention from students of the subject—those of Rittinger and Kick. Rittinger's law states that the power required for reduction is proportional to the increase of surface. Kick's law states, in effect, that the power required varies as the volume or weight. Both of these suggestions cannot be correct, since the Rittinger theory calls for the expenditure of approximately 27 times as much power to reduce 1-in. cubes to 200 mesh as is required by the Kick theory. A series of tests and experiments made by Prof. John W. Bell, of Me Gill University, led to the conclusion that the Rittinger theory was more nearly correct and could be used as a basis for comparisons. A paper by Arthur O. Gates, in the Engineering & Mining Journal, May 24, 1913, gave a practical method of illustrating these comparisons and is reproduced herewith.